Many types of input devices are presently available for performing operations in a computing system, such as buttons or keys, mice, trackballs, joysticks, touch sensor panels, touch screens and the like. Touch screens, in particular, are becoming increasingly popular because of their ease and versatility of operation as well as their declining price. Touch screens can include a touch sensor panel, which can be a clear panel with a touch-sensitive surface, and a display device such as a liquid crystal display (LCD) that can be positioned partially or fully behind the panel so that the touch-sensitive surface can cover at least a portion of the viewable area of the display device. Touch screens can allow a user to perform various functions by touching the touch sensor panel using a finger, stylus or other object at a location dictated by a user interface (UI) being displayed by the display device. In general, touch screens can recognize a touch event and the position of the touch event on the touch sensor panel, and the computing system can then interpret the touch event in accordance with the display appearing at the time of the touch event, and thereafter can perform one or more actions based on the touch event.
There are two known techniques used to capacitively measure touch. The first is a “self-capacitance” method to measure capacitance-to ground, whereby a signal is applied to an electrode. A touch in proximity to the electrode causes signal current to flow from the electrode, through an object such as a finger, to electrical ground.
The second technique used to capacitively measure touch is through mutual capacitance. Mutual capacitance touch screens apply a signal to a driven electrode, which is capacitively coupled to a receiver electrode by an electric field. Signal coupling between the two electrodes is reduced by an object in proximity, which reduces the capacitive coupling. This second technique is called “mutual capacitance” method.
Mutual capacitance touch sensor panels can be formed from a matrix of drive and sense lines of a substantially transparent conductive material such as Indium Tim Oxide (ITO), often arranged in rows and columns in horizontal and vertical directions on a substantially transparent substrate. Drive signals can be transmitted through the drive lines, resulting in signal capacitances at the crossover points (sensing pixels) of the drive lines and the sense lines. The signal capacitances can be determined from sense signals that are generated in the sense lines due to the drive signals. In some touch sensor panel systems, multiple drive lines are stimulated simultaneously to generate composite sense signals in the sense lines. While these systems offer some advantages, conventional multi-stimulus systems can cause difficulties. For example, in a typical multi-stimulus system, different drive lines can introduce different phase delays in the sense signals of a sense channel, which can result in reduced efficiency in processing the sense signals.
Within the context of the second technique, various additional techniques have been used to measure the mutual capacitance between electrodes. Each of these techniques has its own capabilities, limitations, and other characteristics, and associated advantages and disadvantages from standpoints such as performance, speed, complexity, cost, and so forth. Moreover, the question of whether a characteristic of a given technique is deemed to be an advantage or disadvantage may depend on the goals of the system designer. For example, the designer of a relatively small touch screen system with low resolution and requiring only one touch detection at a time may consider a characteristic of a given sensing technique to be advantageous, while a designer of a larger touch screen system requiring high resolution and multiple simultaneous touch capability may consider the same characteristic to be a disadvantage.
United States patent application US 2010/00860593 A1, published Mar. 11, 2010, for PHASE COMPENSATION FOR MULTI-STIMULUS CONTROLLER, discloses an elaborate method to mitigate the phase variation over the panel, while United States patent application US 2011/0084857 A1, published Apr. 14, 2011, for MULTITOUCH TOUCH DEVICE WITH MULTIPLE DRIVE FREQUENCIES AND MAXIMUM LIKELIHOOD ESTIMATION overcomes that problem by using baseband orthogonal frequency division multiplexed (OFDM) tones to excite the panel. However this latter prior art raises other serious problems like the ability to handle large peak to average power ratio (PAPR) of the sense signal that has all the drive signals added up, and therefore to avoid saturation of the sense amplifiers, the dynamic range of these amplifiers need to be increased by 10 to 20 dB or even more compared to single carrier systems, for supporting large number of drive channels. This can result in very significant increase in total current consumption of the sense amplifiers and the subsequent analog signal processing chain, making the product uncompetitive.
Interference rejection is another serious issue involving touch controllers. Single carrier systems tend to have a wider signal bandwidth and need to be frequency agile in order to be sufficiently far from interfering frequencies. However it may not always be possible to find the sweet spot for operation as the signal bandwidth may be too wide to avoid multiple interference tones simultaneously. The signal bandwidth is usually inversely proportional to the integration time of a correlation receiver, and making the signal bandwidth very narrow would lead to long integration time in the receiver, resulting in unacceptably low update rates i.e. frame rate of a touch device. Baseband OFDM systems of prior art have subcarrier frequencies at a spacing of Δf, starting from 0 Hz and ending at half the sampling frequency Fs/2. They require a very large fast Fourier transform (FFT) size of N that is equal to the twice the operational frequency span divided by the frequency resolution i.e. N=Fs/Δf. The operational frequency span needs to cover a very wide spectrum range, always starting from 0 Hz and ending at half the sampling frequency, even though the actual frequency band of operation i.e. frequencies of actually used subcarriers, may usually be a much smaller frequency range closer to the upper end of the operational frequency range of Fs/2, than to 0 Hz. The frequency resolution Δf which is the inverse of the FFT period TFFT, is equal to the tone spacing that needs to be sufficiently small in order to find the required number of clean tones that are sufficiently separated from multiple interference tones that may be present in a system e.g. due to LCD display noise or Charger Noise. Also it is desirable to have the clean tones clustered together so that they have similar group delay over the capacitive touch screen panel. As an example, if only 24 clean tones are spread out over 32 consecutive FFT bins just below 500 kHz, with a 500 Hz tone spacing, the prior art will require an FFT size of 2048 (nearest power of 2 greater than 2*500 kHz/500 Hz) instead of just 32, at a 1024 kHz sampling frequency Fs. Large FFT sizes consume more power and require a higher sampling speed.
It may also be desirable to support both mutual capacitance and self-capacitance at the same time without having to time multiplex between the two modes in order to support the required frame rates for regular touch with a good signal to noise ratio (SNR). Known techniques today do not offer this capability.
Support for passive stylus requires a very high density of drive and sensor electrodes and existing signal processing techniques may be too expensive or consume too much power to support this requirement.
Support for large touch panels will require cheap and high resistance ITO layers that result in large amplitude attenuation over the panel along with large phase shifts. The resistance increases due to the increased length of the ITO electrodes for larger touch panels.